1. Field of the Invention
The present invention relates to a technique for generating a high-precision reference clock signal suitably applied to a data storage system using a rotatable disk-like storage medium and, more particularly, to a phase control circuit for a digital apparatus capable of high-density, high-speed data access. More specifically, the present invention relates to an optical disk drive system capable of high-speed data access, which is suitably used as an external memory unit of a highly-advanced computer.
2. Description of the Related Art
Of various types of data storage disk units, sample-servo scheme optical disk drive system attract a great deal of attention from engineers. This is because such optical disk apparatuses are firmly believed by the engineers to be much superior to the remaining types in a data storage capacity, operation reliability, and application range.
When such a sample-servo scheme optical disk apparatus is to be used as an external memory unit of a current, highly-advanced computer, an improvement in data processing performance of the computer demands a higher data accessing speed of the optical disk drive system while the manufacturing cost is minimized and excellent operation reliability is maintained.
Various technical subjects or obstacles lie in the process of increasing the data accessing speed of an optical disk drive system. A technique of extracting a high-precision reference clock signal is one of them. In general, a reference clock signal for data recording and reproduction is generated in the following manner. Clock pits included in servo byte areas of a rotating optical disk are read by scanning of a light beam spot. A reproduced clock pit pulse signal is supplied to a phase-lock loop control circuit to obtain an oscillation output from a voltage-controlled oscillator. A reference signal generation circuit arrangement for a sample-servo scheme optical disk apparatus is disclosed in, e.g., ISOM '87 Technical Digest "CLOCK JITTER IN SAMPLED SERVO FORMAT" Takashi Takeuchi et al., 1987, pp. 137-140.
If the rotational speed of an optical disk itself is increased in order to increase the data accessing speed of an optical disk drive system, high-frequency jitter components are generated in the resultant reproduced reference clock signal due to an increase in mechanical resonant vibration produced in an optical disk driving mechanism portion. Residual high-frequency jitter components degrade the precision of a reference clock signal and greatly degrade the data accessing performance.
In order to increase the precision of a reference clock signal in a high-speed optical disk drive system, the sampling frequency of a phase-lock loop control circuit may be increased to jack up the reproduction clock rate of an optical disk. If a high sampling frequency is set, the operation range of the phase-lock loop control circuit which is free from high-frequency jitter can be expanded. With an increase in reproduction clock rate, therefore, reproduction of a high-precision reference clock signal can be expected. With such an arrangement, however, the area occupied by servo byte areas including clock pits is increased on the optical disk which is sample-servo formatted in a limited surface area. With this increase in area, effective data storage areas are relatively reduced. In other words, the format efficiency of the optical disk is decreased. This impairs one of the merits of an optical disk, i.e., a large recording capacity.
In order to increase the precision of a reference clock signal in a high-speed optical disk drive system, the mechanical rigidness and process precision of an optical disk driving mechanism portion may be improved to suppress or eliminate the occurrence of mechanical resonance associated with high-speed rotation of an optical disk, which is a direct cause of high-frequency jitter components. Such an improvement in a mechanical system, however, is not welcomed by the manufacturers of optical disk drive system for the following reasons. The above-described mechanical means for solving the problem complicates the mechanical design and the manufacturing process and increases the manufacturing cost.